Measuring and/or monitoring a level of a liquid in a tank or tanks is a challenging task. One method of accomplishing this task is mechanical in nature. A spring biased mass is suspended within the tank and partially immersed in the liquid. Due to buoyancy, the effective weight of the mass is offset by the amount of the liquid held in the tank. The spring hanging in the tank is also magnetically coupled to a pointer through a brass pressure-sealing barrier. The pointer is generally associated with a scale read by sight. While the pointer does give an indication of the level of the liquid in the tank, the pointer is accurate to no better than about plus or minus ten percent (+/−10%). Therefore, the mechanical method of measuring and/or monitoring the level in the tank does not provide a particularly trustworthy reading.
Since the mechanical method was deficient, electrical methods of measuring the level of the fluid in the tank were attempted. For example, the use of a time domain reflectometry (TDR) system was explored. The conventional TDR system includes a probe which is inserted into the tank and immersed in the liquid. A signal generator in the TDR system generates a signal or pulse that propagates along the probe. When the signal reaches the air/liquid interface (i.e., the level of the liquid in the tank which is sometimes referred to as a dielectric mismatch boundary), the signal is reflected back toward a signal receiver. The signal receiver captures the reflected or return signal and transmits characteristics of that return signal to the TDR system. Using those characteristics, the TDR system is able to determine the level of the liquid in the tank.
Despite the success of the conventional TDR system, it was eventually discovered that some liquids commonly found or stored in tanks did not reflect signals very well. For example, liquids with a relatively low dielectric constant (i.e., permittivity) were only able to reflect a weak signal. If the signal was too weak, the TDR system was unable to accurately and/or reliably determine the level of the liquid in the tank. This limited the particular applications where the conventional TDR system was useful.
To solve the weak reflected or return signal problem experienced with low dielectric constant liquids, a modified bistatic radar was used. The radar employed a float coupler moveably disposed on the probe as disclosed in U.S. Pub. Applns. 2004/0046571 and 2004/0046572 to Champion, et al., and U.S. Pub. Appln. 2004/0059508 to Champion. The float coupler was buoyant upon the liquid and configured to reflect the signal by coupling of the two separate but parallel conductors. When the signal generator generated a signal, the signal propagated along the first conductor, passed through the float coupler floating on the surface of the liquid, and then propagated back up to the signal receiver. As a result, the TDR system was able to measure and/or monitor the level of the liquid in the tank despite the low dielectric constant liquid stored in the tank.
The modified bistatic radar employing the float coupler worked very well where the radar was provided ready access to the liquid in the tank. However, the modified bistatic radar was impractical in situations where pressurized and/or compressed low dielectric liquids were found. For example, carbon dioxide (CO2), which has a relatively low dielectric constant of about one and six tenths (1.6), is often stored in a compressed and/or liquefied form in tanks, canisters, and the like, e.g. Dewar-type containers. Carbon dioxide is often used as an inexpensive, nonflammable pressurized gas to carbonate soft drinks and make seltzer, to inflate life jackets, to power paintball guns, to inflate bicycle tires, to oxidize metals in welding, to refrigerate foods, to remove caffeine from coffee, to extinguish fires, to remove oil from the underground, and the like. Indeed, the applications for carbon dioxide extend across a wide range.
High pressure tanks can store the product indefinitely. The insulated or Dewar-type containers hold the product at a lower pressure by having the contents at a low temperature as well. Particular gasses may be more suitable stored in one type of container or the other. By way of example and not of limitation, the following chart illustrates some common industrial liquefied gasses, their typical liquefied container, and the dielectric constant of the liquid gas:
Common Industrial Liquefied GassesLiquid gasTypical Liquefied ContainerDielectric ConstantPropaneHigh Pressure1.6ButaneHigh Pressure1.4CO2High Pressure or Insulated1.6 (1.563)NitrogenInsulated1.43ArgonInsulated1.5ChlorineHigh Pressure2.1OxygenInsulated1.5HydrogenInsulated1.23HeliumInsulated1.05
Unfortunately, because carbon dioxide (and other liquid gasses) has a low dielectric constant and is stored in a compressed and/or pressurized state and in liquefied form, neither the conventional TDR or the improved bistatic radar system was well suited to determine the level of the carbon dioxide in tanks. The standard TDR system would not work well since the carbon dioxide possessed a low dielectric constant. Also, the bistatic radar approach was out since the liquefied carbon dioxide was a pressurized fluid and the tank most likely had only a small opening available for the probe. Trying to fit the probe including a probe coupler into the tank would have been impractical and difficult. Even if the probe and probe coupler could be squeezed through any available small opening, the size of the opening would have placed restrictions on the size of the probe. This would probably negatively affect the operation of the improved bistatic radar system. Moreover, many tanks had internal features on which a moving float coupler would likely get hung up.
Because many customers would like to be able to electrically read the level of the liquefied carbon dioxide (so they can convey that reading via, for example, a radio, phone, and/or network a central billing office, a dispatch office, and the like) an improved apparatus for and method of measuring and/or monitoring the level of liquefied carbon dioxide in a tank would be desirable. The invention provides such an apparatus and method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.